The present invention relates to a plasma addressed liquid crystal display having a flat panel structure in which a liquid crystal cell is laminated on a plasma cell through an intermediate thin glass sheet, and particularly to a technique for preventing deterioration of the liquid crystal cell due to ultraviolet rays generated on the plasma cell side.
A plasma addressed liquid crystal display has been disclosed, for example, in Japanese Patent Laid-open No. Hei 4-265931. FIG. 2 shows a structure of the plasma addressed liquid crystal display. As shown in this figure, the plasma addressed liquid crystal display has a flat panel structure including a liquid crystal cell 1, a plasma cell 2, and a common thin glass sheet 3 interposed therebetween. The thin glass sheet 3 is as extremely thin as about 50 .mu.m in thickness, and is called "a microsheet". The plasma cell 2 has a lower glass substrate 4 joined to the thin glass sheet 3, and an ionizable gas is sealed in a gap therebetween. Stripe-like discharge electrodes are formed on an inner surface of the lower glass substrate 4. These discharge electrodes are composed of anodes A and cathodes K that are alternately arranged with one another. The discharge electrodes can be finely printed and baked on the flat glass substrate 4 at a high productivity and a high workability by screen printing or the like. Partitioning walls 7 are formed on the anodes A of these discharge electrodes for partitioning the gap filled with the ionizable gas into discharge channels 5. The partitioning walls 7 can be also formed and baked by screen printing in such a manner that top portions of the partitioning walls 7 are in contact with one surface side of the thin glass sheet 3. In this way, the discharge channels 5 are surrounded by a horizontal plane of the glass substrate 4, a horizontal plane of the thin glass sheet 3, and vertical side planes of the partitioning walls 7. As described above, the stripe-like discharge electrodes function as alternately arranged anodes A cathodes K, and generate plasma discharge therebetween. The plasma discharge involves generation of ultraviolet rays. In addition, the thin glass sheet 3 is joined to the lower glass substrate 4 by glass frit or the like.
The liquid crystal cell 1 has a transparent upper glass substrate 8. The glass substrate 8 is adhesively bonded on the other surface side of the thin glass sheet 3 using a sealing material or the like with a specific gap formed therebetween. The sealed gap is filled with liquid crystal 9. The liquid crystal 9 is composed of nematic liquid crystal, for example, 90.degree. twist-oriented liquid crystal material. Signal electrodes 10 are formed on an inner surface of the upper glass substrate 8. The signal electrodes 10 are perpendicular to the stripe-like discharge channels 5, and matrix-like pixels are formed at crossing points between the signal electrodes 10 and the discharge channels 5. The flat panel structure having the above configuration is of a transmission type, in which the plasma cell 2 is positioned on the light incident side and the liquid crystal cell 1 is positioned on the light outgoing side. A polarizer 11 is mounted on an outer surface of the plasma cell 2 for converting illuminating light emitted from a backlight 12 into linearly polarized incident light. An analyzer 13 is mounted on an outer surface of the liquid crystal cell 1 for analyzing linearly polarized outgoing light having which has passed through the liquid crystal cell 1.
In the plasma addressed liquid crystal display having the above configuration, display is performed by switching the discharge signal to scan the rows of discharge channels 5 having the function of performing plasma discharge in a line-sequential manner and applying an image signal to the columns of signal electrodes 10 on the liquid crystal cell 1 side in synchronization with the scanning. When plasma discharge is generated in one discharge channel, the interior of the discharge channel comes to be at a nearly constant anode potential, thus performing pixel selection for one row. In other words, the discharge channel 5 functions as a sampling switch. When an image signal is applied to each signal electrode corresponding to the selected pixel in a state in which the plasma sampling switch is made conductive, a relevant pixel is sampled. Thus, lighting-on or lighting-off of the pixel is controlled. After the plasma sampling switch comes to be in a non-conductive state, the signal voltage remains in the pixel. To be more specific, the liquid crystal cell 1 converts polarized incident light into polarized outgoing light on the basis of a signal voltage, to perform image display. For this purpose, the liquid crystal 9 is composed of nematic liquid crystal, for example, twist-oriented. To orient the liquid crystal 9, an alignment film 14 is formed on the inner surface of the upper glass substrate 8, and an alignment film 15 is formed on the surface of the thin glass sheet 3 which surface is in contact with the liquid crystal 9.
In the plasma addressed liquid crystal display, the liquid crystal cell 1 is addressed by scanning the discharge channels 5 on the plasma cell 2 side. Upon scanning of the discharge channels 5, plasma discharge is generated. At this time, the plasma discharge involves generation of ultraviolet rays. In the related art plasma addressed liquid crystal display, ultraviolet rays Bring generated by plasma discharge have been made directly incident in the liquid crystal cell 1 through the intermediate thin glass sheet 3. The liquid crystal cell 1 contains organic materials constituting the liquid crystal 9, the alignment films 14 and 15, and the like. In general, ultraviolet rays cause deterioration of the organic materials constituting the liquid crystal 9 and the alignment films 14 and 15. This reduces a voltage retention ratio of the liquid crystal 9 and/or changes a pre-tilt angle of the twist-oriented liquid crystal 9. As a result, there occur problems of an after-image phenomenon due to unevenness in voltage retention ratio within a screen and of a variation in voltage/transmittance characteristic of the liquid crystal.